Secondary clarifiers with large areas are widely applied in wastewater treatment plants. A pilot study was conducted to
examine the possibility of applying one and two-stage inclined tube settlers instead of conventional secondary clarifiers. Tube
diameter in the first stage of the two-stage settler was wide as the conventional ones, but in the second stage, it was narrow to
improve the efficiency. The results indicated that in short detention times, the tube settler was more effective in shorter
detention time than the conventional secondary sedimentation basin, and its effluent of TSS and turbidity was acceptable to
discharge into the surface waters. The average removal of TSS, BOD5, and COD, in a 20-minute detention time in the tubes, in
the one-stage tube settler pilot plants was 97.6%, 96.4%, and 96.36%, respectively, while in the conventional secondary
sedimentation basin was 98.2%, 99%, and 98.6%, respectively. There was a good agreement between theoretical analyses and
experimental results of the pilot plant. Two-stage tube settlers in the series could improve hydraulic condition and removal
efficiency of TSS, in comparison with the one-stage tube settler. The average TSS removal, in shorter detention times than that
the one-stage, was 97.8%.